Cationic photopolymerisation was developed with the intention of permitting the polymerisation, under light irradiation, of monomers that cannot be polymerised by a radical pathway, for example epoxy resins.
Photopolymerisation of epoxy resins by a cationic pathway was thus primarily developed in the field of paints, coatings, and adhesives. However, cationic photopolymerisation suffers from slower polymerisation rates and lower final degrees of conversion than those obtained by means of polymerisation via a radical pathway. Drying epoxy-based paints or curing epoxy-based parts may thus take from about 10 minutes (min) to several hours (h).
In addition, photopropagation within the thickness of coatings to be polymerised, and thus the photopolymerised thicknesses obtained, are more limited with the cationic pathway because of the limited number of potential initiators and monomers, and also because of the complexity of polymerisation mechanisms.
Thus, cationic photopolymerisation is more suitable for thin products and/or products with a low filler content and/or products that are not highly colored. Cationic photopolymerisation has become highly developed, starting from the academic and industrial work by J. V. Crivello who discovered the family of onium salts as photoinitiators (see the following publications: J. V. Crivello, T. P. Lockart, and J. L. Lee: Journal of Polymer Science Polymer Chemistry, Edition 21, 97-109 (1983), studying the thermal decomposition of iodonium and sulfonium salts with the addition of heat; J. V. Crivello: Advances in Polymer Science. 62, 1-48 (1984), studying iodonium and sulfonium salts as photoinitiators).
This family of photoinitiators includes iodonium, sulfonium, phosphonium and pyridinium salts.
Iodonium and sulfonium salts are the most widely used. Phosphonium salts are difficult to use because of their toxicity. Pyridinium salts are more complete photoinitiators because they can be used alone to initiate a cationic polymerisation by irradiation or thermally, but with heating of the salt in order to destabilize it and cause it to decompose, the heating temperature being higher than 40° C.; it may be up to 120° C. These latter salts have been developed and studied by Y. Yagci (see publications: Y. Yagci and T. End. Advances in Polymer Science 127, 59-86 (1997), studying pyridinium salts as a photoinitiator or thermal initiator; Y. Yagci and I. Reetz, Progress in Polymer Science 23, 1485-1538 (1998), studying pyridinium salts as a photoinitiator or thermal initiator).
Cationic polymerisation via a thermal pathway, in particular of epoxies, is rather limited because of the small number of initiators that are available. Epoxy resins are usually polymerised by amines as the principal or secondary initiator (co-initiator).
Initiator systems composed of acid anhydrides or indeed thiols are also known. These initiator systems, namely amine, acid anhydride, and thiols, result in polymerisation of the anionic type or in polycondensation. The structure of the polymer obtained by polycondensation is very different from structures obtained by an anionic or cationic pathway. With polycondensation, a three-dimensional (3D) network is constituted by polymer chains connected together via nitrogen-type bridges. Thus, its nature is more that of a copolymer than a homopolymer, in particular an epoxy. With cationic and anionic polymerisation, a 3D network may be generated with cross-linking ties of the same chemical nature as the polymer chains. A polyether matrix is formed thereby.
In order to be able to polymerise larger thicknesses than those obtained by photopolymerisation, hybrid initiator systems have been developed that involve two different chemistries, namely that of epoxies and that of urethanes, for example.
Initiator systems are also known that can be used for photochemical polymerisation followed by a thermal pathway using heat.
EP 0 066 543 relates to polymerisable compositions comprising epoxy monomers (A) polymerised by adding external heat to said compositions, i.e. by heating, in the presence of a catalyst (B) and a co-catalyst (C). The catalyst (B) or initiator comprises a quaternary ammonium salt, in particular an aromatic N-heterocyclic compound. Under the effect of heat, the co-catalyst (C) generates a radical that reduces the catalyst (B) in a redox reaction, producing a by-product that initiates the polymerisation reaction by reaction with the monomer (A). Without adding heat, and thus at ambient temperature, a polymerisation reaction cannot take place.
Thus, there is a need for a cationic polymerisation initiator system that can be used to combine photopolymerisation and/or polymerisation via a thermal pathway in the presence of a co-initiator, without adding heat, and that can be used for polymerisation via a photochemical pathway at the surface and via a thermal pathway in the core of the layer to be polymerised as a function of said layer, regardless of whether it is filled and/or pigmented and/or includes reinforcement. Systems of this type are known as dual-cure cationic systems.
The term “dual-cure” as used in the context of the present invention means any system that involves two polymerisation processes, i.e. a photochemical process and a thermal process (in particular via the exothermicity of the reaction). The term “dual-cure” means that the chemistries of polymerisation by a photochemical pathway and by a thermal pathway are similar. When there are two chemistries that are different, for example a radical chemistry and a cationic chemistry, or indeed an epoxy chemistry and a urethane chemistry, for example, that is termed hybrid polymerisation.
Thus, the present invention relates to a composition that can be polymerised by a dual-cure cationic pathway, using the same chemistry issuing from the same initiator for photochemical initiation and/or thermal initiation as a function of the thickness and transparency of the coating to be polymerised (which may optionally be filled and/or pigmented).